Descriptions

Total organic carbon (TOC) content of marine sediments represents residual carbon, originally derived
from terrestrial and marine sources, which has survived seafloor and shallow subseafloor diagenesis.
Ultimately, its preservation below the sulfate reduction zone in marine sediments drives methanogenesis.
Within the gas hydrate stability zone (GHSZ), methane production along continental margins
can supersaturate pore fluids and lead to the formation of gas hydrate. In this paper we examine the
inventory and sources of TOC in sediments collected from four regions within the GHSZ along the Indian
continental margins. The recovered sediments vary in age from Oligocene to recent. Mean TOC abundance
is greatest in the KrishnaeGodavari (KeG) Basin and decreases progressively to the Mahanadi
basin, Andaman wedge, and KeralaeKonkan (KeK) Basin. This decrease in TOC is matched by a progressive
increase in biogenic CaCO3 and increasing distance from terrestrial sources of organic matter
and lithogenic materials. Organic carbon sources inferred from C/N and d13CTOC range from terrestrial (K
eG Basin) to mixed marine and terrestrial (Mahanadi Basin), to marine dominant (Andaman wedge and
KeK Basin). In the KeG Basin, variation in the bulk d13CTOC is consistent with changes in C3 and C4
vegetation driven by monsoon variability on glacial-interglacial timescales, whereas in the Mahanadi
Basin a shift in the d13CTOC likely reflects the onset of C4 plant deposition in the Late Miocene. A large shift
the d13CTOC in the KeK basin is consistent with a change from C3 to C4 dominated plants during the
middle Miocene. We observe a close relationship between TOC content and gas hydrate saturation, but
consider the role of sedimentation rates on the preservation of TOC in the zone of methanogenesis and
advective flow of methane from depth. Although TOC contents are sufficient for in situ methanogenesis at
all the sites where gas hydrates were observed or inferred from proxy data, seismic, borehole log,
pressure core, and gas composition data coupled with relatively high observed gas hydrate saturations
suggest that advective gas transport may also play a role in the saturation of methane and the formation
of gas hydrates in these regions. Although TOC content may be a first order indicator for gas hydrate
potential, the structural and stratigraphic geologic environment along a margin will most likely dictate
where the greatest gas hydrate saturations will occur.

The financial support for the NGHP01, from the Oil IndustryDevelopment Board, Oil and Natural Gas Corporation Ltd., GAIL(India) Ltd. and Oil India Ltd. is gratefully acknowledged. We alsoacknowledge the support extended by all the participating organizationsof the NGHP: MoP&NG, DGH, ONGC, GAIL, OIL, NIO, NIOT,and RIL. This research was funded by the Directorate General of Hydrocarbons(DGH) India and the U.S. Geological Survey (Contract #07CRSA0708). M.E.T. acknowledges support from a fellowship fromthe Institute for Advanced Study (HWK), Delmenhorst, Germany.